In the process of drilling oil wells, segments of rock, clay or the like (hereafter drill cuttings or drilling waste) are created by the drilling process and are carried to the surface by drilling fluid circulating in the well. Drill cuttings generally range in size from fines (approximately 1 to 100 microns) to rock chips (approximately 1 to 2 cm). As is known, in addition to carrying drilling waste to the surface, the drilling fluid serves other purposes including strengthening the walls of the wellbore, preventing contamination of the well and damage to the various formations, protecting metal parts from corrosion, providing lubrication to the drilling string as well as cooling and lubricating the drill bit during drilling.
The drilling fluids used in drilling a well are often a hydrocarbon based slurry commonly referred to as an “oil mud”. An oil mud is generally comprised of a high proportion of oil based fluids together with other additives that are designed to impart specific properties to the drilling fluid. Drilling fluids are often expensive fluids that constitute a significant expense of a drilling program. Moreover, as drill cuttings are carried to the surface, they will absorb hydrocarbons or will otherwise become coated with hydrocarbons contained in the oil mud and from hydrocarbons released from various formations. As a result of this contamination, the drilling waste is unfit for simple disposal at the surface. Hydrocarbon/drilling fluid contamination of the drill cuttings may be approximately 10-50% of the total volume of the drill cuttings off the drilling rig shaker system. Accordingly, at the surface, drill cuttings and the drilling fluid are subjected to various separation techniques (most commonly shakers and/or centrifuges) in order to recover as much drilling fluid as possible for re-use in the well and to stabilize the drill cuttings for disposal.
In addition, hydrocarbon contaminated drill cuttings must be stabilized and/or cleaned before disposal in order to mitigate environmental damage and comply with government regulations.
However, most drill cutting/drilling fluid separation technologies only provide a preliminary separation of drill cuttings and drill fluid with the end result being that substantial amounts of hydrocarbons (typically 10-50% of the total volume of the drill cuttings) from the drilling fluid and the formations remain coated on the drill cuttings after rudimentary surface separation. As noted above, as both drilling fluid and other hydrocarbons are valuable and government regulations require either cleaning of drill cuttings or special containment at a disposal site, there has been a need for improved techniques to recover a greater percentage of drilling fluid and hydrocarbons from drill cuttings and to provide cleaner drill cuttings that can be readily disposed of.
A review of the prior art reveals that various technologies for cleaning drill cuttings have been described. Canadian Patent No. 2,317,858 and U.S. Pat. No. 6,550,552 to Pappa et al. disclose a washing process in which drill cuttings contaminated with an oil-based drilling fluid are successively washed using ethyl acetate or hexane. U.S. Pat. No. 5,755,892 to Herold et al. discloses washing drill cutting with ecologically compatible, biologically degradable oil. U.S. Pat. No. 4,645,608 to Rayborn discloses separating oil contaminated cuttings from a drilling mud, contacting the cutting with a detergent solution to remove the oil from the cuttings and returning the oil and detergent solution to the drilling mud. U.S. Pat. No. 4,942,929 to Malachosky et al. discloses removing drill cuttings from a well and sequentially passing cuttings through a shale shaker, washing with water and disposing of the cuttings. U.S. Pat. No. 6,846,420 to Reddy et al. discloses introducing drill cuttings into a separating zone, adding an aqueous acidic solution containing a polymer substituted with an amino group and a halogenating agent such as sodium hypochlorite (“bleach”) and U.S. Pat. No. 5,199,997 to Stowe discloses a first inclined tub containing a heated stripper solution, a second inclined tub containing a hot rinse liquid and a third inclined tub containing cold rinse water for removing oil from drill cuttings.
In drill cutting cleaning processes, the cuttings are often agitated to aid in the removal of hydrocarbon fluids from the cuttings. This agitation generally degrades the cuttings into smaller sized particles and fines, adding to the difficulty of separating the cuttings from hydrocarbon fluids and often leaving small fragments of cuttings or fines in recovered drilling fluids. The properties of a drilling fluid are important for the effectiveness of the drilling fluid, and fines in a recovered fluid can alter the properties of the fluid and reduce its' effectiveness. Therefore it is desirable to prevent degradation of the cuttings during cleaning.
Furthermore, it is important that the costs of cleaning contaminated drill cuttings are reasonable and/or are improved over past techniques.